Today, essentially all daily newspapers, and a substantial portion of other publications, are printed using lithographic printing plates. Such plates are sometimes referred to as "planographic" printing plates because both the printing and the non-printing areas of the plate lie in substantially the same plane. Thus, in place of the raised and lowered ink accepting surfaces found in letterpress and gravure printing plates, respectively, lithographic printing plates possess oleophilic ink receptive areas and hydrophilic non-ink receptive areas. The hydrophilic areas, which are readily wetted by water, tend to repel oil-based printing inks and correspond to the background non-printed areas of the image. The oleophilic areas, on the other hand, tend to attract oil-based inks and correspond to the printed areas of the image to be printed.
Lithographic printing plates of the type most commonly used by the daily newspaper industry typically comprise an aluminum substrate, an anodized aluminum layer on the surface of the substrate, and a photosensitive coating that is applied to the surface of the anodized aluminum layer opposite to the aluminum substrate.
The predominant method of imaging lithographic printing plates of the type described above involves exposing the photosensitive coating of the plate to light (e.g., actinic radiation) through a suitable photographic mask which bears a negative or positive of the image to be printed and which may include both continuous (text) and half-tone (picture) portions. The plate is then developed with a suitable developer to remove the photo-sensitive coating from the background areas of the image thereby uncovering the anodized surface of the plate in those areas. The developer may also act to enhance the oleophilic character of the photosensitive coating which remains in the areas of the plate which were exposed through the mask. Because this method requires the preparation of a photographic mask corresponding to the original work, and the lithographic plate is imaged from the mask rather than from the original work, it is considered to be an "indirect" imaging method.
More recently, direct imaging methods which do not require a photographic mask have been used commercially. In one such method, the photosensitive coating of a plate ike that described above is exposed to a computer controlled laser beam rather than to light through a photographic mask. The computer controls the laser based upon information electronically stored in digitized form. In another method, the photosensitive coating is made of a photoconductive material that is first electrostatically charged and then selectively discharged by exposure to a computer controlled laser beam. Toner is applied to the electrostatic image thus formed on the plate. After toning, the plate is developed to remove the photosensitive layer from the untoned areas thereby exposing the anodized aluminum layer of the plate in those areas.
Thus, a number of lithographic plate imaging techniques have been proposed in the art. Of those techniques, the ones that have been most successful commercially have been characterized by: (a) the use of a photosensitive layer which is selectively exposed to light; and (b) the removal of a portion of the photosensitive layer in a development step. Because the photosensitive materials used in such techniques typically are relatively expensive and the removal of portions of such layers during the preparation of imaged lithographic printing plates is a time-consuming expensive process, the development of simpler, less expensive techniques is highly desirable. Particularly desirable is the development of alternative commercially practicable techniques for making imaged lithographic printing plates that can be used to make more than 50,000 or even 100,000 or more prints of good quality.